Efferent insights into cochlear mechanics

Abstract

Medial olivocochlear efferent (MOCE) effects on tone-evoked basilar membrane (BM) vibrations are analyzed in terms of the vector differences between BM responses with and without MOCE stimulation. The technique is sensitive to rapid (i.e., "fast", τ≤100ms) changes in both the amplitude and the phase of the BM responses, and reveals MOCE effects over much wider frequency and intensity ranges (for a given BM location) than previously envisioned. The findings confirm and extend previous suggestions that MOCE effects are brought about by at least two different, outer-hair-cell based mechanisms. The effects on BM responses to characteristic frequency (CF) tones are consistent with suggestions that the MOCEs affect both the stiffness and the damping of the cochlear partition (damping effects dominating at low levels, and stiffness effects dominating at high levels). The analyses also indicate that MOCE activity can enhance, rather than inhibit, BM responses to low frequency tones (well below CF) - albeit by miniscule amounts. If one assumes that all of the mechanical effects of MOCE activation are brought about via gain changes in a single "cochlear amplifier" [4], these results seem to reveal that this amplifier exhibits a frequency-dependent transition from negative feedback (below CF) to positive feedback (near CF). This scenario is reminiscent of the type of amplification proposed by Mountain et al. a long, long time ago [9]. © 2011 American Institute of Physics.